EFFICIENT CONTROL SYSTEM FOR ELECTRIC MOTORS

Abstract: An electric power controller (8) for use with AC induction motors (9) that measures line voltage and current phase angles, and uses a microprocessor (10) to calculate delay values for firing silicon control rectifier (SCR) gates (76) to provide efficient amounts of current to the motors (9). The calculation is made using the measured phase angles, an inputted measured full load power factor for each motor, and corrected power factor value stored in a read-only-memory (ROM).

Description: EFFICIENT CONTROL SYSTEM FOR ELECTRIC MOTORS FIELD OF THE INVENTION The field of this invention is electric current control for motors. More specifically, this invention provides an improved efficient control system for alternating current (AC) induction motors with automatic control of electric current, whereby the minimum amount of electric current required for mechanical power is provided. Automatic adjustment is made by the controller of the invention to assure that provided electric currents are at minimum levels required for motors to function without damage. BACKGROUND OF THE INVENTION AC induction motors are well known in the prior art and generally, unless otherwise controlled, operate at approximately constant speeds within certain performance envelopes. Such operation is independent of both the magnitude of supplied voltages and mechanical loads that may be coupled to die motors. Furthermore, without a controller this type of motor in general uses about the same amount of electric current whether loaded or unloaded. Therefore, in actual applications, these motors have certain inherent inefficiencies. Because supplied line voltages normally fluctuate, AC induction motors must be selected so as to generate adequate mechanical power over the entire range of anticipated voltage fluctuation. If a motor is selected that generates adequate mechanical power at the nώiimum expected voltage, then excess mechanical power is generated as the line voltage increases, and under such conditions electric power is wasted. Likewise in situations requiring variable mechanical power from AC induction motors the selection must be made so that motors will generate adequate power to match maximum loads even at minimum line voltage. Therefore, when less than maximum power is required, such selected motors will be generating excess power, which unavoidably wastes electric current. In situations with both variable loads and voltages these two dynamic parameters can combine to increase wasted electric current. Prior art has discussed various control systems for electric motors that attempt to reduce the amount of electric current that is unnecessarily used by motors. For example, see U.S. Patent No. 4,052,648 to Nola, issued October 4, 1977. The Nola patent shows a controller alleged to be usable with a single-phase motor. Nola discloses that if, e.g., a three-phase motor is to be used one of the disclosed control systems must be connected to each phase input to the motor. AC induction motors, if operated at improper current levels, can be damaged. Safe operating current levels are a function of both provided line voltage and coupled mechanical loads. Therefore, practical and effective control systems must not only reduce currents to more efficient and economical levels but must also provide safe operating current levels to avoid motor damage. Power factor is a quantity used to describe operation of AC electric motors. It is a function of the phase relationship between supplied electric voltage and current. To quantify the power factor, in a percentage format a ratio must be taken of the cosine of the angle zero which equals one over the cosine of the phase angle between the voltage and current. This phase angle is often symbolized by the Greek letter theta. Under ideal conditions, current and voltage are "in phase," i.e., theta equals zero, and the power factor is 100 percent. In most practical applications, current and voltage are out of phase; theta is thus greater than zero and power factors of less than 100 percent occur. Electric motors known in the art, fully loaded, typically have power factors of 80 percent or better. Such motors, lightly loaded, may have power factors of only 40 percent or less. These low power factors increase current flows, and also inefficiently increase electricity costs for motor operation. SUMMARY OF THE INVENTION The motor controller of the present invention controls the amount of AC current supplied to induction motors within every single cycle so that AC power can be minimized according to operating conditions. This circumstance maintains increased motor power factors so the phase angle between current and voltage, i.e., theta, closely approximates zero. Currents needed to operate motors therefore are reduced, and accordingly so are costs for electricity to operate motors. The present invention constitutes a fully digitized power controller for AC induction motors of one, two or three phases. The controller unit is designed to control the AC power consumed by the motor through a programmed microprocessor which regulates each power phase by changing the firing angle (i.e., the delay between the transition of current and voltage which is the phase angle, theta) of silicon control rectifiers (SCR's) to compensate for less-than-fully-loaded conditions. Regulation is effected from a single signal simultaneously provided to a SCR for each power phase. This controller effectively reduces to minimum levels the electric current fed to motors, while still providing adequate required mechanical power, at any given moment, for imposed workloads and available line voltage. The controller also acts to turn motors off when any of the following fault conditions occur: excessive operating temperatures; SCR failure; power phase loss; or power phase reversal. To activate the controller a full load power factor for the motor to be operated must be determined and that value must be input to a microprocessor in the controller. Inputting of the measured value is accomplished through a digital switch. The microprocessor using this value, which is the core of the Controller, can then calculate with reference to a look-up table the required current firing delay value. This value is used to regulate motor operation through firing of SCR's or other devices, known in the art, to accomplish the same result such as thyristors. The microprocessor continuously controls the phase of input voltages, and currents, and also monitors SCR heat sink temperatures. Based on these measurements the controller directs changes in firing angles for the SCR's to ensure operation at optimum efficiency. Counters are used for monitoring operating voltage and current phase angles by reference to zero crossings for these parameters. In the preferred embodiment, the controller computes percentage of energy savings which are continuously displayed on a readout. In case of a fault, the readout signals appropriately to alert operators. The controller is designed for industrial use and is intended to be installed by a knowledgeable electrician who first operates a motor, without the controller, at full load to measure the power factor for the motor to be operated. It is this determined power factor that is inputted through digital switches and utilized by the microprocessor for continuous control of the electrical current provided to the motor. BRIEF DESCRIPTION OF THE DRAWINGS The drawings depict a preferred embodiment of the controller of this invention for a three-phase AC induction motor as follows: Figure 1 is a block circuit schematic drawing of the electrical circuit for the controller of the invention; Figure 2 is a block circuit schematic for a preferred embodiment of the controller connected to an external magnetic contactor and motor; Figure 3 is a circuit schematic for the output driver shown in Fig. 1; and Figure 4 is a logic flow chart for software according to the invention for the microprocessor shown in Fig. 1. DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention comprises a controller, generally designated by reference number 8, having a microprocessor 10 for operating motors 9 at more energy-efficient and hence more economical rates. The controller 8 embodiment described here is for a three-phase AC induction motor 9 that is to be used with an external contactor. While the controller 8 is described here by reference to the preferred embodiment, it will be readily understood that numerous modifications and substitutions could be made without departing from the invention. For example, the components could be hardwired instead of using printed circuit boards (PCB's). Similarly, components and their relative arrangements may be varied. The microprocessor 10 may take any of the various known forms, so long as it contains an internal program that monitors the phase relationship of voltage and current inputs and retards or advances trigger impulses used to control SCR's that in turn make necessary phase relationship adjustments. Voltage and current phase relationships can continuously change and hence to provide power for efficient operation this phase relationship needs to be readjusted automatically as applied mechanical loads and line voltages vary. While the controller 8 embodiment described here is for a three-phase motor, those skilled in the art will readily understand the design of a comparable controller for one or two-phase motor operation. The internal program may be devised in numerous different forms and modes so long as it contains the logic steps set forth in Figure 4, in the order given. Shown in Figure 1 is the microprocessor 10 that is connected to an oscillator 12 containing a 12-megaHertz clock used for the central processor unit (CPU) clock. A counter 14 serves to enable the clock contained in oscillator 12 to function as a clock for other counters as described below. Current signals from each of the three phases, LI, L2 and L3 (see Figure 1), for the motor 9 are input to AC input opto couplers 96 that are connected across output SCR modules, 76, 78, and 80. The AC input opto couplers 96 are used to detect the zero crossing of each of three phases. The three current signals are each shaped by Schmitt trigger circuits 18 and then sent to NOR gate 20. The output of NOR gate 20 is sent to an 8 bit binary counter 22, for which the clock in oscillator 12, as divided by 2048 in counter 14, serves as a clock signal. As soon as the phase current output from NOR gate 20 drops to zero, counter 22 begins to count and send its output to digital 8 bit comparator 24. So long as the phase currents are not zero, the output of NOR gate 20 will be at the high level and the counter 22 will be brought into a reset state. Once the NOR gate 20 output becomes zero and the output of the counter 22 passes to the comparator 24, it acts to compare this information with the motor's current fire delay value obtained from register 28. This register 28 stores the current fire delay value which the microprocessor 10 calculated based on the measured motor power factor input when operation of the controller 8 was initiated, as discussed in detail below. The microprocessor 10, using measurements of zero crossings for the voltage and current, refers to a look-up table in an internal read only memory (ROM) to determine the amount of time delay required to effectively retard gating signals. Data stored in the internal ROM is so determined as to identify actual ultimate full load power factors for the motor 9 being controlled. Such determinations are made using techniques known to those skilled in the art. The comparator 24 therefore compares the output of running counter 22 with the fixed value of the calculated current fire delay value stored in register 28. When the output of counter 22 is less than the output of register 28, the signal from comparator 24 is zero. After the outputs of counter 22 and register 28 equalize the digital comparator 24 outputs a positive signal that is applied to a pulse generator 32 implemented by. a Schmitt NAND gate 30. The pulse generator 32 includes an oscillator circuit having a capacitor 34, two identical but oppositely connected signal rectifier diodes 36 and two resistors 38 and 39, resistor 38 being of approximately four times the value of resistor 39. Open drain inverters 40 are used as a wire OR circuit to handle "picket fencing" of motor 9 operation and allow the microprocessor to control the motor 9 by gating SCR pulses. The output of the pulse generator 32 is split and simultaneously passed through three drivers 42. The identical signals from the drivers 42 then each pass to Darlington transistors 44 used to simultaneously fire gates of corresponding SCR's through the output drivers 82, 84 and 86. Zero voltage crossing values for the three power phases are detected using three AC input opto couplers 48 connected in a wye configuration. The opto couplers 48 isolate the controller 8, for safety reasons, from external voltages. After passage through Schmitt triggers 18 where the signals are shaped, the outputs of Schmitt triggers 18 are passed to AND gate 52. The output of AND gate 52 is three times the AC input frequency and is applied to the input of a retriggerable monostable multi-vibrator 54 having a time constant determined by resistor and capacitor 56. The retriggerable monostable multi¬ vibrator 54 is chosen to catch any power phase loss into the controller 8 input. In combination with resistor and capacitor 56 the monostable multi-vibrator 54 acts to effect interruption when any one or more of the power phases is not operating. A second retriggerable monostable multi-vibrator 58 is connected directly to the output of one of the Schmitt triggers 18 from its opto coupler 48 and serves to indicate the exact time when the phase of the voltage input, in this case power phase 1, crosses the zero level. At that time a short pulse from dual retriggerable monostable multi-vibrator 58 resets counter 60 to immediately commence counting from 0. The clock of oscillator 12, being divided by 2048 through counter 14, acts as the clock for counter 60. If the current for the power phase connected to the phase 1 input drops to zero, the signal from the output of AND gate 52 write the counter 60 value into register 62 and an external interrupt from NOR gate 64 is issued. This value is used as information data by the programmed microprocessor 10 and is read from register 62 through a common data bus 63. When the controller 8 is set or reset, the required power factor for the connected motor is input using dual inline package (DIP) digital switches 66 that are then read through buffer 68. The first of eight switches for digital switch 66 is connected to a selected soft start operation; the next three are used to set the soft-start ramp time; and the remaining switches are connected to input the measured power factor for a fully loaded motor. This power factor must be determined by the operator using a power factor meter, as are known in the art, and input before initiating operation of the controller 8. In operation the controller calculates the percentage of energy savings and displays these values on multi-segment LED displays 72 that are driven by decoder-drivers 74. When a fault is detected, the LED displays 72 flash, e.g., all "8"'s, to alert the operator. Turning to Figure 2, it shows the controller 8 in a typical installation with an induction motor 9, external magnetic contactor 118, and die control circuit for the contactor 120. The contacts of relay 29 of the controller 8 are intended to be wired to the control circuit 120 of the external magnetic contactor 118. When the controller 8 is functioning properly and no faults are detected, the microprocessor 10 outputs a signal to driver 19 energizing relay 29, closing its contacts allowing the motor 9 to be turned on. If a fault is detected, die contacts of relay 19 will open and die motor will be turned off. If power is detected on input terminal LI, L2 or L3 before relay 29 is energized, the controller 8 will not turn on. The SCR modules are generally referenced by numbers 76, 78 and 80, for each of the three power phases. Each SCR module 76, 78 and 80 is respectively connected to one of the output drivers 82, 84 and 86 which are identical and are detailed in figure 3. Figure 3 shows the electrical circuit schematic for each of the three SCR driver circuits 82, 84 and 86, which are all identical. In this circuit a pulse transformer 90 is provided a signal through resistor 92 from its Darlington drive transistor 44. Rectifier diode 94 is provided for suppression. As also shown in Figure 3, the output from one of the pulse transformer 90's secondaries passes d rough serially connected resistor 98 and rectifier diode 100 into the gate of the SCR. The negative gate voltage is clamped by die diode 102, across die secondary winding. Terminal 1 is connected to the gate of the SCR and terminal 2 is connected to the cathode of the SCR. The output of the other pulse transformer 90 secondary passes dirough serially connected rectifier diode 106 and resistor 104 into the gate of the other SCR of the pair. The negative gate voltage is clamped by die diode 108 across die secondary winding. Terminal 4 is connected to die gate of the SCR and terminal 3 is connected to die cadiode of the SCR. Conduction between the cathode and gate is needed to trigger SCR firing. A metal oxide varistor 110 is connected across die power terminals of me SCR's, which can be Thyristors, for surge suppression and a resistor 112 and capacitor 114 are also connected across the SCR's in a conventional manner to stabilize their operation (Dv/Dt Suppression). The light emitting diode (LED) side of AC input opto coupler 96 is connected across the power terminals of the SCR's in series with resistor 97. When die SCR's are on, there is no current flowing through the LEDs and die output transistor of the opto coupler 96 is off. When die SCR's are off, current flows dirough resistor 97 and die LEDs of the opto coupler 96 turn die output transistor on. Figure 4, is self explanatory to those knowledgeable in die art. It sets forth logic of the algorithm used for microprocessor 10. As can be seen, the controller 8 of this invention is designed so that its microprocessor 10 operates without external memory in an essentially four-port configuration. One port operates as a data bus for input-output peripherals. Another port is a control bus to access peripheral integrated circuits. Another port serves as the input for phase voltage and current measurements, and temperature data from a controller heat sink, while the last of the ports has the auxiliary function of providing input to external interrupts and a serial link. In die described preferred embodiment, die microprocessor 10 is an INTEL 8051. As will be readily apparent, many modifications of tiiis invention can be made by tiiose skilled in die art without departing from its spirit and scope. It is intended tiierefore that the invention should be limited only by the appended claims.

Claims: What is claimed is: 1. An electric power controller for AC induction motors comprising : measuring means adapted to measure input electric current and voltage phase angle values; microprocessor means adapted to receive a measured full load power factor value for said motor, and to read corrected power factor value data from a read-only-memory (ROM) means; electric current control means adapted to change measured input phase angle values for electric current supplied to said motor; whereby said microprocessor means calculates a delay for operation of said current control means to conduct current to said motor based on (i) measured input electric current and voltage phase angle values, (ii) the inputted full load power factor value, and (iii) corrected power factor values read from said ROM means, and said microprocessor outputs die calculated delay to operate said current control means. 2. An electric power controller according to claim 1, wherein said measuring means includes counter means to provide signals for measuring phase angle values based on electric current and voltage zero crossings. 3. An electric power controller according to claim 1, wherein said current control means includes silicon control rectifier (SCR) means, each SCR means including a gate means used to control conduction of electric current. 4. A metiiod for controlling electric current provided to AC induction motors, including die steps of: measuring electric current and voltage phase angle values; inputting to a microprocessor means a measured full load power factor value for said motor; inputting corrected power factor value data to a read-only-memory (ROM) means; calculating using a microprocessor means a delay value for operation of an electric current control means adapted to change measured input phase angle values for electric current supplied to said motor; wherein said microprocessor means calculates the delay value based on (i) measured input electric current and voltage phase angle values, (ii) die inputted full load power factor value, and (iii) corrected power factor values read from said ROM means.

Inventors: ROEMER JAY STEWART, ZILBERMAN YEVGENY, ROEMER, JAY, STEWART, ZILBERMAN, YEVGENY
Applicants: ECONELECTRIC NORTH AMERICA, ECONELECTRIC OF NORTH AMERICA, INC.
Application year: 1994
Main classification: H02P5
IPC codes: H02J3, H02P23, H02P25, H02P27